Users often use client devices to request, receive and process content from content servers over networks (e.g., retrieving web pages from web sites via the Internet). Content servers can identify individual users and provide personalized content to the users. For example, users can set up accounts with the content servers. The users can then provide their account login credentials to the content servers before or in connection with content requests, and the content servers can provide personalized content, such as content tailored to the user's prior browsing history (e.g., if the content server is a news web site, new articles can be highlighted and articles that the user has previously read can be de-emphasized). However, this requires users to create accounts with each content server providing such personalized content, and users must typically log into the content servers periodically or during each browsing session. In addition, the content servers must maintain information regarding the user accounts.
As an alternative to storing account-level information on content servers and requiring users to create accounts with content servers, the content servers may store information (e.g., “cookies”) on the client devices. The information may be stored on the client devices and provided back to the content servers with subsequent content requests so that the content servers can, e.g., identify the user and/or device. Thus, if the same user subsequently uses the same client device to request content from the same content server, the content server can identify the user and provide personalized or customized content without requiring the user to log into the content server.